The tumor microenvironment: a double-edged sword

Differentiation in neuroblastoma: diffusion-limited hypoxia induces neuro-endocrine secretory protein 55 and other markers of a chromaffin phenotype

Background

Neuroblastoma is a childhood malignancy of sympathetic embryonal origin. A high potential for differentiation is a hallmark of neuroblastoma cells. We have previously presented data to suggest that in situ differentiation in tumors frequently proceeds along the chromaffin lineage and that decreased oxygen (hypoxia) plays a role in this. Here we explore the utility of Neuro-Endocrine Secretory Protein 55 (NESP55), a novel member of the chromogranin family, as a marker for this process.

Methodology/Principal Findings

Immunohistochemical analyses and in situ hybridizations were performed on human fetal tissues, mouse xenografts of human neuroblastoma cell lines, and on specimens of human neuroblastoma/ganglioneuroma. Effects of anaerobic exposure on gene expression by cultured neuroblastoma cells was analyzed with quantitative real-time PCR. Fetal sympathetic nervous system expression of NESP55 was shown to be specific for chromaffin cell types. In experimental and clinical neuroblastoma NESP55 immunoreactivity was specific for regions of chronic hypoxia. NESP55 expression also correlated strikingly with morphological evidence of differentiation and with other chromaffin-specific patterns of gene expression, including IGF2 and HIF2α. Anaerobic culture of five neuroblastoma cell lines resulted in an 18.9-fold mean up-regulation of NESP55.

Conclusions/Significance

The data confirms that chronic tumor hypoxia is a key microenvironmental factor for neuroblastoma cell differentiation, causing induction of chromaffin features and NESP55 provides a reliable marker for this neuronal to neuroendocrine transition. The hypoxia-induced phenotype is the predominant form of differentiation in stroma-poor tumors, while in stroma-rich tumors the chromaffin phenotype coexists with ganglion cell-like differentiation. The findings provide new insights into the biological diversity which is a striking feature of this group of tumors.

Synthesis, Crystal Structure, and Rotational Energy Profile of 3-Cyclopropyl-1,2,4-benzotriazine 1,4-Di-N-oxide

1,2,4-Benzotriazine 1,4-di-N-oxides are potent antitumor drug candidates that undergo in vivo bioreduction leading to selective DNA damage in the low oxygen (hypoxic) cells found in tumors. Tirapazamine (TPZ) is the lead compound in this family. Here we report on the synthesis, crystal structure, and conformational analysis of a new analog, 3-cyclopropyl-1,2,4-benzotriazine 1,4-di-N-oxide (3). Compound 3 (C(10)H(10)N(3)O(2)) crystallized in the monoclinic space group C2/c. Unit cell parameters for 3: a = 16.6306 (12), b = 7.799 (5), c = 16.0113 (11) Å, α = 90, β = 119.0440 (10), γ = 90, and z = 8.

Suicide gene therapy on spontaneous canine melanoma: correlations between in vivo tumors and their derived multicell spheroids in vitro

To validate the use of multicellular spheroids to predict the efficacy of herpes simplex thymidine kinase/ganciclovir (HSVtk/GCV) suicide gene therapy in the respective in vivo tumors, we established and characterized 15 melanoma-derived cell lines from surgically excised melanoma tumors. Three HSVtk-lipofected cell lines were not sensitive to GCV in any culture configuration, other five displayed similar sensitivity as monolayers or spheroids, and only one resulted more sensitive when grown as spheroids. Other six cell lines manifested a relative multicellular resistance (MCR) phenotype growing as spheroids, compared with the same cells growing as monolayers. The reverse correlation between the MCR and the monolayers survival to HSVtk/GCV suggests that one of the main causes of MCR would be the rapid cell repopulation after suicide gene treatment. The high correlation of MCR with the spheroids radial growth and with the mitotic index of the respective originary tumors supported this re-growth involvement. A remarkable finding was the high correlation in HSVtk/GCV sensitivity between in vivo tumor and the corresponding derived cell lines growing as spheroids (R(2) = 0.85). This strongly encourages the implementation of spheroids as highly realistic experimental model for optimizing and predicting the in vivo response of the respective tumors to therapeutic strategies.

Initiation of DNA strand cleavage by 1,2,4-benzotriazine 1,4-dioxide antitumor agents: mechanistic insight from studies of 3-methyl-1,2,4-benzotriazine 1,4-dioxide

The antitumor agent 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine, TPZ, 1) gains medicinal activity through its ability to selectively damage DNA in the hypoxic cells found inside solid tumors. This occurs via one-electron enzymatic reduction of TPZ to yield an oxygen-sensitive drug radical (2) that leads to oxidatively generated DNA damage under hypoxic conditions. Two possible mechanisms have been considered to account for oxidatively generated DNA damage by TPZ. First, homolysis of the N-OH bond in 2 may yield the well-known DNA-damaging agent, hydroxyl radical. Alternatively, it has been suggested that elimination of water from 2 generates a benzotriazinyl radical (4) as the ultimate DNA-damaging species. In the studies described here, the TPZ analogue 3-methyl-1,2,4-benzotriazine 1,4-dioxide (5) was employed as a tool to probe the mechanism of DNA damage within this new class of antitumor drugs. Initially, it was demonstrated that 5 causes redox-activated, hypoxia-selective oxidation of DNA and small organic substrates in a manner that is completely analogous to TPZ. This suggests that 5 and TPZ damage DNA by the same chemical mechanism. Importantly, the methyl substituent in 5 provides a means for assessing whether the putative benzotriazinyl intermediate 7 is generated following one-electron reduction. Two complementary isotopic labeling experiments provide evidence against the formation of the benzotriazinyl radical intermediate. Rather, a mechanism involving the release of hydroxyl radical from the activated drug radical intermediates can explain the DNA-cleaving properties of this class of antitumor drug candidates.

Spectral imaging facilitates visualization and measurements of unstable and abnormal microvascular oxygen transport in tumors

Abnormal microvasculature contributes to the pathophysiologic microenvironment of tumors. Understanding microvascular tumor oxygen transport is necessary to comprehend the factors that influence tumor biology, physiology, and therapy. Previously, we described an in vivo spectral imaging microscopy system for measurements of microvessel hemoglobin saturation (HbSat). We measure temporal fluctuations and spatial gradients in tumor microvessel oxygenation and identify instances of anastomoses between vessels with significantly different oxygenations. Slow periodic fluctuations in HbSat <0.2 cycles per minute were observed. These measurements are consistent with microelectrode measurements of fluctuating tumor oxygenation. Gradients in HbSat along individual tumor microvessels were measured that were larger in magnitude than normal tissue microvessels. Images were captured of anastomoses of tumor microvessels with diameters <or=100 microm and significantly different HbSat values (>20%). Shunting of inspired oxygen, presumably due to arteriovenous anastomoses, from tumor feeding arterioles to adjacent venules was imaged. This effect was confined to a region around the tumor and was not observed in nearby normal microvessels. Imaging measurements of tumor microvessel oxygen transport may offer insight to current questions regarding oxygen-related tumor biology and treatment responses, and spectral imaging may be a useful research tool in this regard.

DNA strand damage product analysis provides evidence that the tumor cell-specific cytotoxin tirapazamine produces hydroxyl radical and acts as a surrogate for O(2)

The compound 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine, TPZ) is a clinically promising anticancer agent that selectively kills the oxygen-poor (hypoxic) cells found in solid tumors. It has long been known that, under hypoxic conditions, TPZ causes DNA strand damage that is initiated by the abstraction of hydrogen atoms from the deoxyribose phosphate backbone of duplex DNA, but exact chemical mechanisms underlying this process remain unclear. Here we describe detailed characterization of sugar-derived products arising from TPZ-mediated strand damage. We find that the action of TPZ on duplex DNA under hypoxic conditions generates 5-methylene-2-furanone (6), oligonucleotide 3'-phosphoglycolates (7), malondialdehyde equivalents (8 or 9), and furfural (10). These results provide evidence that TPZ-mediated strand damage arises via hydrogen atom abstraction from both the most hindered (C1') and least hindered (C4' and C5') positions of the deoxyribose sugars in the double helix. The products observed are identical to those produced by hydroxyl radical. Additional experiments were conducted to better understand the chemical pathways by which TPZ generates the observed DNA-damage products. Consistent with previous work showing that TPZ can substitute for molecular oxygen in DNA damage reactions, it is found that, under anaerobic conditions, reaction of TPZ with a discrete, photogenerated C1'-radical in a DNA 2'-oligodeoxynucleotide cleanly generates the 2-deoxyribonolactone lesion (5) that serves as the precursor to 5-methylene-2-furanone (6). Overall, the results provide insight regarding the chemical structure of the DNA lesions that confront cellular repair, transcription, and replication machinery following exposure to TPZ and offer new information relevant to the chemical mechanisms underlying TPZ-mediated strand cleavage.

The influence of tumour microenvironmental factors on the efficacy of cisplatin and novel platinum(IV) complexes

The chemotherapeutic drug cisplatin is an important treatment for many types of solid tumours, in particular non-small cell lung cancer (NSCLC). Platinum(IV) complexes offer several advantages to cisplatin due to their requirement for reduction to the active platinum(II) form to elicit cytotoxicity. This should minimise non-specific effects and facilitate higher amounts of the active complexes reaching the target DNA. Hypoxia and a quiescent cell population are features of the tumour microenvironment known to lead to resistance to many chemotherapeutic agents. It is unclear how these microenvironmental factors will impact on the efficacy of novel platinum(IV) complexes. Consequently, the cytotoxicities of several platinum drugs were determined in monolayer and tumour spheroid cultures derived from NSCLC lines. Platinum(IV) reduction potential correlated well with cytotoxicity. The complex containing a chloro axial ligand demonstrated the greatest potency and the drug with the hydroxy ligand was the least effective. Although drug cytotoxicity was not enhanced under hypoxic conditions, both cisplatin and the platinum(IV) complexes retained full potency. In addition, all of the platinum drugs retained the ability to evoke apoptosis in quiescent cells. In summary, unlike many anticancer drugs, the platinum(IV) complexes retain cytotoxic potency under resistance-inducing tumour microenvironmental conditions and warrant further investigation as more selective alternatives to current platinum-based therapy for the treatment of solid tumours.

A model of quiescent tumour microregions for evaluating multicellular resistance to chemotherapeutic drugs

The quiescent cell population of tumours poses a barrier to the success of many cancer therapies. Most chemotherapeutic drugs target proliferating cells, but the growth fraction of many tumours is low. Based on the multicellular tumour spheroid model, a system was developed using human colon adenocarcinoma (DLD-1) cells to mimic the microenvironment of quiescent microregions of solid tumours. The quiescent tumour spheroids (TS(Q)) showed decreased expression of the proliferation marker Ki-67 and increased expression of the quiescence marker p27(kip1) compared to proliferating spheroids (TS(P)). The quiescent status of the TS(Q) was confirmed by long-term growth assessment. The quiescence was completely reversible demonstrating that the TS(Q) retained the ability to proliferate and morphological assessment by light microscopy confirmed the absence of significant apoptosis. When the efficacy of widely used chemotherapeutic drugs was determined, vinblastine, doxorubicin, cisplatin and 5-fluorouracil (5-FU) all produced significant cell death in the TS(P). However, while still effective, the potencies of doxorubicin and cisplatin were significantly reduced in TS(Q). In contrast, 5-FU and vinblastine did not produce cell death in the TS(Q). In summary, TS(Q) show considerable resistance to a panel of established chemotherapeutic agents and represent a useful model for evaluating the efficacy of drugs and other cancer therapies in quiescent tumours.

Radiotherapy alone or combined with carbogen breathing for squamous cell carcinoma of the head and neck

BACKGROUND

The current study was performed to investigate the efficacy of carbogen (95% oxygen [O(2)] and 5% carbon dioxide [CO(2)]) breathing during definitive radiotherapy (RT) to enhance local control.

METHODS

Between November 1996 and November 2002, 101 patients with previously untreated T2 to T4 squamous cell carcinomas of the oropharynx, larynx, and hypopharynx were entered onto a prospective trial and randomized to receive definitive hyperfractionated RT alone or combined with carbogen breathing. Patients were stratified according to T classification and primary tumor site. Follow-up ranged from 1-91 months (median, 38 months). All living patients had follow-up for more than 2 years. Outcomes analyses were performed according to intent to treat.

RESULTS

Definitive RT alone was completed as planned in 50 of 51 patients (98%); 49 of the 50 patients (98%) who were randomized to receive carbogen breathing were able to complete the RT as planned. Three patients randomized to receive carbogen breathing declined carbogen. The 5-year outcomes after RT alone or combined with carbogen were as follows: local control, 83% versus 88% (P = 0.5155); locoregional control, 81% versus 83% (P = 0.7174); distant metastasis-free survival, 82% versus 86% (P = 0.5184); cause-specific survival, 73% versus 77% (P = 0.5866); and absolute survival, 53% versus 58% (P = 0.4856).

CONCLUSIONS

The addition of carbogen breathing to definitive RT did not appear to improve the likelihood of local control significantly. However, because of the limited size of the current study, the authors cannot definitively conclude that carbogen breathing is ineffective.

Utility of 19F MRS detection of the hypoxic cell marker EF5 to assess cellular hypoxia in solid tumors

BACKGROUND AND PURPOSE

The present studies were undertaken to determine whether 19F MRS could be used to quantify the binding of the pentafluorinated derivative of etanidazole (EF5) in hypoxic cells of solid tumors.

MATERIALS AND METHODS

A 4.7 T imaging magnet was used for the in situ and in vitro evaluation of EF5 signals. In order to develop a better understanding of these NMR measurements the characteristics of parent, reduced unbound, and reduced bound EF5 signals were examined in vitro using a 12 T spectrometer.

RESULTS

In situ data acquired using a 4.7 T imaging magnet, showed retention of EF5 signals in KHT sarcomas that was absent in muscles for 6 h after EF5 injection. In vitro studies showed no difference in the NMR detectable signal of parent and reduced unbound EF5. T2 values determined using parent EF5 samples revealed a T2 time of 675 ms. In contrast, EF5 bound to KHT tumor cells gave rise to signals of low intensity, broad line widths, and T2 relaxation times of less than 30 ms. When the same samples were analyzed using the 4.7 T imaging magnet, the CF3 and CF2 fluorine peaks were readily identifiable in the parent EF5 sample but no fluorine signal could be detected from EF5 bound to KHT tumor cells.

CONCLUSION

The inability to resolve bound EF5 metabolites even at high field strengths (12 T), coupled with the short T2 relaxation times of the bound EF5, and the limits of detection of the in situ applied imaging magnet (4.7 T), meant that hypoxic cells could not be quantified in tumors using the 19F MRS technique. In situ 19F MRS measurements of EF5 signals (parent/reduced unbound) may reflect conditions of tumor physiology and thus indicate the extent of tumor hypoxia but they are not capable of resolving the cellular oxygenation status of the tumor cells.

Enzyme-Activated, Hypoxia-Selective DNA Damage by 3-Amino-2-quinoxalinecarbonitrile 1,4-Di-N-oxide

The compound 3-amino-2-quinoxalinecarbonitrile 1,4-dioxide (4) displays potent hypoxia-selective cytotoxicity in cell culture. This compound is structurally similar to the known hypoxia-selective DNA-damaging agent tirapazamine (1, TPZ), but the ability of 4 to cause DNA damage under low-oxygen conditions has not previously been characterized. The results presented here provide the first evidence that 4 causes reductively activated DNA damage under hypoxic conditions. The findings indicate that one-electron reduction of 4 by NADPH:cytochrome P450 reductase yields an oxygen-sensitive intermediate (5). This activated intermediate is rapidly destroyed by reaction with O2 under aerobic conditions, but goes forward to cause DNA damage under low-oxygen conditions. Analysis of the DNA damage indicates that reductive activation of 4 leads to production of a highly reactive, freely diffusible oxidizing radical that causes sequence-independent cleavage of the deoxyribose backbone and oxidative damage to the heterocyclic bases in duplex DNA. On the basis of the experiments reported here, the chemical nature of the DNA damage caused by redox-activated 4 is analogous to that reported previously for TPZ.

DNA base damage by the antitumor agent 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine)

Tirapazamine is a bioreductively activated DNA-damaging agent that selectively kills the hypoxic cells found in solid tumors. This compound shows clinical promise and is currently being examined in a variety of clinical trials, including several phase III studies. It is well established that DNA is an important cellular target for tirapazamine; however, the structural nature of the DNA damage inflicted by this drug remains poorly understood. As part of an effort to understand the chemical events responsible for the hypoxia-selective cytotoxicity of this drug, the studies reported here are designed to characterize tirapazamine-mediated damage to the genetic information stored in the heterocyclic base residues of double-stranded DNA. Here, we used gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry to characterize and quantify oxidative DNA base damage mediated by tirapazamine. A multiplicity of modified bases including 8,5'-cyclopurine-2'-deoxynucleoside tandem lesions were identified and quantified. The results provide the first detailed insight regarding the structural identity of the DNA base lesions caused by this drug. Interestingly, it appears that the hypoxic conditions under which tirapazamine operates, along with the unique chemical properties of the drug, yield a unique variety of DNA base damage that is dominated by formamidopyrimidine and 5-hydroxy-6-hydropyrimidine lesions. Importantly, the results suggest that tirapazamine may generate a set of poorly repaired, potentially cytotoxic DNA base lesions that block DNA transcription and replication. Overall, the results indicate that DNA base damage may contribute to the biological effects of tirapazamine in vivo.

Evidence for hypoxia-induced neuronal-to-chromaffin metaplasia in neuroblastoma

We present evidence that in neuroblastoma, a pediatric malignancy of embryonal sympathetic origin, hypoxia, underlies a phenotypic switch from a primitive neuronal to a chromaffin cell type. This conclusion is based on morphological and molecular data on 116 clinical tumors and is supported by data on the phenotypic effects of hypoxia on neuroblastoma cell lines when studied in monolayer culture and as tumor xenografts. In the clinical material, extensive chromaffin features were seen in regions of chronic tumor hypoxia. This was the exclusive form of intra-tumoral maturation of stroma-poor tumors and was also seen in stroma-rich tumors, either exclusively or in combination with ganglion-like cells. In neuroblastoma cell lines, hypoxia induced changes in gene expression associated with the chromaffin features observed in vivo. We therefore propose tumor hypoxia as a major cue determining phenotype in sympathetic tumors of neuroblastic origin. Because it appears to be reversible upon reoxygenation in monolayer culture, we suggest the term metaplasia for the phenomenon.

Impact of the p53 status of the tumor cells on the effect of reactor neutron beam irradiation, with emphasis on the response of intratumor quiescent cells

Human head and neck squamous cell carcinoma cells transfected with mutant p53 (SAS/mp53) or with neo vector as a control (SAS/neo) were inoculated subcutaneously into both the hind legs of Balb/cA nude mice. Tumor-bearing mice received 5-bromo-2'-deoxyuridine (BrdU) continuously to label all proliferating (P) cells in the tumors. After administration of sodium borocaptate-10B (BSH) or p-boronophenylalanine-10B (BPA), the tumors were irradiated with neutron beams. The tumors not treated with 10B-compound were irradiated with neutron beams or gamma-rays. The tumors were then excised, minced and trypsinized. The tumor cell suspensions thus obtained were incubated with a cytokinesis blocker, and the micronucleus (MN) frequency in cells without BrdU labeling (=quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. Meanwhile, 6 h after irradiation, tumor cell suspensions obtained in the same manner were used for determining the frequency of apoptosis in Q cells. The MN and apoptosis frequencies in total (P+Q) tumor cells were determined from the tumors that were not pretreated with BrdU. Without 10B-carriers, in both tumors, the relative biological effectiveness of neutrons was greater in Q cells than in total cells, and larger for low than high cadmium ratio neutrons. With 10B-carriers, the sensitivity was increased for each cell population, especially for total cells. BPA increased both frequencies for total cells more than BSH. Nevertheless, the sensitivity of Q cells treated with BPA was lower than that of BSH-treated Q cells. These sensitization patterns in combination with 10B-carriers were clearer in SAS/neo than in SAS/mp53 tumors. The p53 status of the tumor cells had the potential to affect the response to reactor neutron beam irradiation following 10B-carrier administration.

Significance of the response of quiescent cell populations within solid tumors in cancer therapy

In analyzing the response of quiescent (Q) cells in solid tumors, we have developed a combined method with a micronucleus (MN) assay and the identification of proliferating (P) cells by 5-bromo-2'-deoxyuridine (BrdU) and an anti-BrdU monoclonal antibody. Using this method, the responses of Q tumor cells as well as total tumor (P + Q) cells within murine solid tumors to various DNA-damaging treatments were evaluated. Based on this evaluation, combining with tirapazamine, a well-known bioreductive agent, and/or heat treatment at mild temperatures was thought to be a promising modality for cancer therapy in terms of conventional anticancer treatment-resistant Q cell control. Recently, our method for detecting the Q-cell response using P cell labeling with BrdU and the MN frequency assay was also shown to be applicable to an apoptosis detection assay. Meanwhile, our method for detecting the intratumor Q-cell response was also applicable toward high linear energy transfer radiation, including reactor neutrons. Thus, using our method, a new neutron capture compound that has the potential to be distributed in neutron capture therapy-resistant intratumor Q cell populations is now under development.

Radiobiological characteristics of solid tumours depending on the p53 status of the tumour cells, with emphasis on the response of intratumour quiescent cells

Human head and neck squamous cell carcinoma cells transfected with mutant TP53 (SAS/mTP53) or with a neo vector as a control (SAS/neo) were inoculated subcutaneously (s.c.) into both hind legs of Balb/cA nude mice. Mice bearing tumours received 5-bromo-2'-deoxyuridine (BrdU) continuously to label all proliferating (P) cells in the tumours. The mice then received gamma-ray irradiation. Another group of mice received a series of test doses of gamma-rays while alive or after tumour clamping to obtain hypoxic fractions (HFs) in the tumours. Right after irradiation, the tumour cells were isolated and incubated with a cytokinesis blocker. The micronucleus (MN) frequency in the cells without BrdU labelling (=quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. Meanwhile, 6 h after irradiation, tumour cell suspensions obtained in the same manner were used for determining the frequency of apoptosis in the Q cells. The MN frequency and apoptosis frequency in total (P+Q) tumour cells were determined from the tumours that were not pretreated with BrdU. In total cell populations, SAS/mTP53 cells were more radioresistant than SAS/neo cells in clonogenic survival. Q tumour cells exhibited a significantly lower apoptosis and MN frequency, probably due to their much larger HF, than total cells. In both total and Q cell fractions, SAS/mTP53 cells were less susceptible to apoptosis and more susceptible to micronucleation than SAS/neo cells. Obviously, TP53 status had the potential to influence the radiosensitivity of not only the total cells, but also the Q cells. However, irrespective of the TP53 status, significant differences in radiosensitivity between total and Q tumour cells were consistently observed. From the viewpoint of tumour control as a whole, including intratumour Q tumour cell control, a treatment modality for enhancing the Q cell response has to be considered.

Evaluation of apoptosis and micronucleation induced by reactor neutron beams with two different cadmium ratios in total and quiescent cell populations within solid tumors

PURPOSE

Response of quiescent (Q) and total tumor cells in solid tumors to reactor neutron beam irradiation with two different cadmium (Cd) ratios was examined in terms of micronucleus (MN) frequency and apoptosis frequency, using four different tumor cell lines.

METHODS AND MATERIALS

C57BL mice bearing EL4 tumors, C3H/He mice bearing SCC VII or FM3A tumors, and Balb/c mice bearing EMT6/KU tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. Thirty min after i.p. injection of sodium borocaptate-10B (BSH), or 3 h after oral administration of p-boronophenylalanine-10B (BPA), the tumors were irradiated with neutron beams. The tumors without 10B-compound administration were irradiated with neutron beams or gamma-rays. This neutron beam irradiation was performed using neutrons with two different Cd ratios. The tumors were then excised, minced, and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the MN frequency in cells without BrdU labeling (=Q cells) was determined using immunofluorescence staining for BrdU. Meanwhile, for apoptosis assay, 6 h after irradiation, tumor cell suspensions obtained in the same manner were fixed, and the apoptosis frequency in Q cells was also determined with immunofluorescence staining for BrdU. The MN and apoptosis frequencies in total (P + Q) tumor cells were determined from the tumors that were not pretreated with BrdU.

RESULTS

Without 10B-compounds, the sensitivity difference between total and Q cells was reduced by neutron beam irradiation. Under our particular neutron beam irradiation condition, relative biological effectiveness (RBE) of neutrons was larger in Q cells than in total cells, and the RBE values were larger for low Cd-ratio than high Cd-ratio neutrons. With 10B-compounds, both frequencies were increased for each cell population, especially for total cells. BPA increased both frequencies for total cells more than BSH did. Nevertheless, the sensitivity of Q cells treated with BPA was lower than that of Q cells treated with BSH. Whether based on the MN frequency or the apoptosis frequency, similar results concerning the sensitivity difference between total and Q cells, the values of RBE, and the enhancement effect by the use of 10B-compound were obtained.

CONCLUSION

Apoptosis frequency, as well as the MN frequency, can be applied to our method for measuring the Q cell response to reactor neutron beam irradiation within solid tumor in which the ratio of apoptosis to total cell death is relatively high, as in EL4 tumor. The absolute radiation dose required to achieve the same endpoint for Q cells is much higher than that for total cells when combined with 10B-compound, especially with BPA.

Evaluation of the potential of p-boronophenylalaninol as a boron carrier in boron neutron capture therapy, referring to the effect on intratumor quiescent cells

C57BL mice bearing EL4 tumors and C3H / He mice bearing SCC VII tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. Three hours after oral administration of l-p-boronophenylalanine-(10)B (BPA), or 30 min after intraperitoneal injection of sodium borocaptate-(10)B (BSH) or l-p-boronophenylalaninol (BPA-ol), a newly developed (10)B-containing alpha-amino alcohol, the tumors were irradiated with thermal neutron beams. For the combination with mild temperature hyperthermia (MTH) and / or tirapazamine (TPZ), the tumors were heated at 40 degrees C for 30 min immediately before neutron exposure, and TPZ was intraperitoneally injected 30 min before irradiation. The tumors were then excised, minced and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling ( = quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. Meanwhile, 6 h after irradiation, tumor cell suspensions obtained in the same manner were used for determining the apoptosis frequency in Q cells. The MN and apoptosis frequency in total (P + Q) tumor cells were determined from tumors that were not pretreated with BrdU. Without TPZ or MTH, BPA-ol increased both frequencies most markedly, especially for total cells. However, as with BPA, the sensitivity difference between total and Q cells was much larger than with BSH. On combined treatment with both MTH and TPZ, this sensitivity difference was markedly reduced, similarly to when BPA was used. MTH increased the (10)B uptake of all (10)B-compounds into both tumor cells. BPA-ol has good potential as a (10)B-carrier in neutron capture therapy, especially when combined with both MTH and TPZ.

Electrochemical treatment of mouse Ehrlich tumor with direct electric current

Electrochemical treatment of cancer utilizes direct electric current (DEC) to produce direct alterations and chemical changes in tumors. However, the DEC treatment is not established and mechanisms are not well understood. In vivo studies were conducted to evaluate the effectiveness of DEC on animal tumor models. Ehrlich tumors were implanted subcutaneously in sixty male BALB/c mice. When the tumor volumes reached 850 mm(3), four platinum electrodes were inserted into the tumors. DEC of 4 mA was applied for 21 min to the treated group; the total charge was 5 C. The healthy and sick control groups were subjected to the same conditions but without DEC. Hematological and chemical parameters as well as histopathological and peritumoral findings were studied. After the electrochemical therapy it was observed that both tumor volume decrease and necrosis percentage increase were significant in the treated group. Moreover, 24 h after treatment an acute inflammatory response, as well as sodium ion decrease, and potassium ion and spleen weight increase were observed in this group. It was concluded that both electrochemical reactions (fundamentally those in which reactive oxygen species are involved), and immune system stimulation induced by cytotoxic action of the DEC could constitute the most important antitumor mechanisms.

Radiobiologic significance of apoptosis and micronucleation in quiescent cells within solid tumors following gamma-ray irradiation

PURPOSE

To determine the frequency of apoptosis in quiescent (Q) cells within solid tumors following gamma-ray irradiation, using four different tumor cell lines. In addition, to assess the significance of detecting apoptosis in these cell lines.

METHODS AND MATERIALS

C3H/He mice bearing SCC VII or FM3A tumors, Balb/c mice bearing EMT6/KU tumors, and C57BL mice bearing EL4 tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. The mice then received gamma-ray irradiation at a dose of 4--25 Gy while alive or after tumor clamping. Immediately after irradiation, the tumors were excised, minced, and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (= Q cells) was determined using immunofluorescence staining for BrdU. Meanwhile, 6 hours after irradiation, tumor cell suspensions obtained in the same manner were fixed. The apoptosis frequency in Q cells was also determined with immunofluorescence staining for BrdU. The MN and apoptosis frequency in total (P + Q) tumor cells were determined from the tumors that were not pretreated with BrdU.

RESULTS

In total cells, SCC VII, FM3A, and EMT6/KU cells showed reasonable relationships between MN frequency and surviving fraction (SF). However, fewer micronuclei were induced in EL4 cells than the other cell lines. In contrast, a comparatively close relationship between apoptosis frequency and SF was found in total cells of EL4 cell line. Less apoptosis was observed in the other cell lines. Quiescent tumor cells exhibited significantly lower values of MN and apoptosis frequency probably due to their large hypoxic fraction, similar to total tumor cells on clamped irradiation.

CONCLUSION

gamma-ray irradiation induced MN formation in SCC VII, FM3A, and EMT6/KU tumor cells, and the apoptosis was marked in EL4 cells compared with the other cell lines. Our method for detecting the Q cell response to gamma-ray irradiation using P cell labeling with BrdU and the MN frequency assay was also applicable to apoptosis detection assay.

Improvement of tumor oxygenation by mild hyperthermia

There is now abundant evidence that oxygenation in rodent, canine and human tumors is improved during and for up to 1-2 days after heating at mild temperatures. An increase in tumor blood perfusion along with a decline in the oxygen consumption rate appears to account for the improvement of tumor oxygenation by mild hyperthermia. The magnitude of the increase in tumor pO(2), determined with oxygen-sensitive microelectrodes, caused by mild hyperthermia is less than that caused by carbogen breathing. However, mild hyperthermia is far more effective than carbogen breathing in increasing the radiation response of experimental tumors, probably because mild hyperthermia oxygenates both (diffusion-limited) chronically hypoxic and (perfusion-limited) acutely hypoxic cells, whereas carbogen breathing oxygenates only the chronically hypoxic cells. Mild hyperthermia is also more effective than nicotinamide, which is known to oxygenate acutely hypoxic cells, in enhancing the radiation response of experimental tumors. The combination of mild hyperthermia with carbogen or nicotinamide is highly effective in reducing the hypoxic cell fraction in tumors and increasing the radiation response of experimental tumors. A primary rationale for the use of hyperthermia in combination with radiotherapy has been that hyperthermia is equally cytotoxic toward fully oxygenated and hypoxic cells and that it directly sensitizes both fully oxygenated and hypoxic cells to radiation. Such cytotoxicity and such a radiosensitizing effect may be expected to be significant when the tumor temperature is elevated to at least 42-43 degrees C. Unfortunately, it is often impossible to uniformly raise the temperature of human tumors to this level using the hyperthermia devices currently available. However, it is relatively easy to raise the temperature of human tumors into the range of 39-42 degrees C, which is a temperature that can improve tumor oxygenation for up to 1-2 days. The potential usefulness of mild hyperthermia to enhance the response of human tumors to radiotherapy by improving tumor oxygenation merits continued investigation.

Change in oxygenation status in intratumour total and quiescent cells following gamma-ray irradiation, tirapazamine administration, cisplatin injection and bleomycin treatment

C3H/He mice bearing SCC VII tumours received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. The mice then received gamma-ray irradiation, or administration of tirapazamine (TPZ), cisplatin or bleomycin. At various time points after each treatment, tumour-bearing mice were irradiated with a series of test doses of gamma-rays, while alive or after being killed, to obtain hypoxic fractions (HFs) in the tumours. Immediately after gamma-ray test irradiation, the tumours were excised, minced and trypsinized. Tumour cell suspensions obtained were incubated with cytochalasin-B, a cytokinesis blocker, and the micronucleus (MN) frequency in cells without BrdU labelling (i.e. quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. MN frequency in the total (P + Q) tumour cells was determined from the tumours that were not pre-treated with BrdU. MN frequency of BrdU-unlabelled cells was then used to calculate the surviving fraction of the unlabelled cells from the regression line for the relationship between the MN frequency and the surviving fraction of total tumour cells. TPZ and cisplatin reduced the HF after treatment, especially in Q cells, and this tendency was particularly marked with TPZ. In contrast, bleomycin increased the HF after treatment. Both reoxygenation following gamma-ray irradiation or bleomycin treatment and a subsequent return to pre-treatment levels of HF following TPZ or cisplatin treatment (rehypoxiation) occurred more rapidly in total (P + Q) cells than in Q cells. Based on our previous report that total (P + Q) and Q cells within this tumour have large acutely and chronically HFs, respectively, we conclude that acute hypoxic cells play a major role in reoxygenation and rehypoxiation in SCC VII tumours.

Reaction of the hypoxia-selective antitumor agent tirapazamine with a C1'-radical in single-stranded and double-stranded DNA: the drug and its metabolites can serve as surrogates for molecular oxygen in radical-mediated DNA damage reactions

The compound 3-amino-1,2,4-benzotriazine 1,4-dioxide (1, tirapazamine; also known as SR4233, WIN 59075, and tirazone) is a clinically promising anticancer agent that selectively kills the oxygen-poor (hypoxic) cells found in tumors. When activated by one-electron enzymatic reduction, tirapazamine induces radical-mediated oxidative DNA strand cleavage. Using the ability to generate a single deoxyribose radical at a defined site in an oligonucleotide, we recently provided direct evidence that, in addition to initiating the formation of DNA radicals, tirapazamine can react with these radicals and convert them into base-labile lesions [Daniels et al. (1998) Chem. Res. Toxicol. 11, 1254-1257]. The rate constant for trapping of a C1'-radical in single-stranded DNA by tirapazamine was shown to be approximately 2 x 10(8) M(-1) s(-1), demonstrating that tirapazamine can substitute for molecular oxygen in radical-mediated DNA strand damage reactions. Because reactions of tirapazamine with DNA radicals may play an important role in its ability to damage DNA, we have further characterized the ability of the drug and its metabolites to convert a C1'-DNA radical into a base-labile lesion. We find that tirapazamine reacts with a C1'-radical in double-stranded DNA with a rate constant of 4.6 x 10(6) M(-1) s(-1). The mono-N-oxide (3) stemming from bioreductive metabolism of tirapazamine converts the C1'-radical to an alkaline-labile lesion more effectively than the parent drug. Compound 3 traps a C1'-radical in single-stranded DNA with a rate constant of 4.6 x 10(8) M(-1) s(-1) and in double-stranded DNA with a rate constant of 1.4 x 10(7) M(-)(1) s(-)(1). We have also examined the rate and mechanism of reactions between the C1'-radical and representatives from two known classes of "oxygen mimetic" agents: the nitroxyl radical 2,2,6, 6-tetramethylpiperidin-N-oxyl (4, TEMPO) and the nitroimidazole misonidazole (5). TEMPO traps the C1'-radical in single-stranded DNA (7.2 x 10(7) M(-1) s(-1)) approximately 3 times less effectively than tirapazamine, but 2 times as fast in double-stranded DNA (9.1 x 10(6) M(-1) s(-1)). Misonidazole traps the radical in single- (6. 9 x 10(8) M(-1) s(-1)) and double-stranded DNA (2.9 x 10(7) M(-1) s(-1)) with rate constants that are roughly comparable to those measured for the mono-N-oxide metabolite of tirapazamine. Finally, information regarding the chemical mechanism by which these compounds oxidize a monomeric C1'-nucleoside radical has been provided by product analysis and isotopic labeling studies.